This invention relates in general to steam generators or boilers and more particularly to a feedwater heater and feedwater heating process for a steam generator.
Natural gas represents a significant source of electrical energy in the United States. It burns with few emissions, and is available throughout much of the country. Moreover, the plants which convert it into electrical energy are efficient and, in comparison to hydroelectric projects and coal-fired plants, they are relatively easy and inexpensive to construct. In the typical plant, the natural gas burns in a gas turbine, causing the rotor of the turbine to revolve and power an electrical generator to which the rotor is connected. The exhaust gasesxe2x80x94essentially carbon dioxide and steamxe2x80x94leave the gas turbine at about 1200xc2x0 F. and themselves represent a significant source of energy. To harness this energy, the typical combined cycle, gas-fired, power plant also has a heat recovery steam generator (HRSG) through which the hot exhaust gases pass to produce steam which powers a steam turbine which, in turn, powers another electrical generator. The exhaust gases leave the HRSG at temperatures as low as 150xc2x0 F.
The steam turbine and the HRSG operate within a loop that also contains a condenser and a feedwater pump. The steam generated by the HRSG passes through the turbine and then into the condenser where it is condensed back into liquid water. The pump delivers that water to the HRSG at about 100xc2x0 F. or perhaps a lower temperature. The water enters the HRSG at a feedwater heater or economizer which elevates its temperature for subsequent conversion into steam within an evaporator and superheater that are also part of the HRSG.
Natural gas contains traces of sulfur, and during the combustion the sulfur combines with oxygen to produce oxides of sulfur. Moreover, the combustion produces ample quantities of water in the form of steam. If the exhaust gases remain above the dew point for the gases, which is about 140xc2x0 F., the oxides of sulfur pass out of the HRSG and into a flue. However, the low temperature feedwater has the capacity to bring the tubes at the downstream end of the feedwater heater below the dew point of the water in the exhaust gases, and when this occurs, water condenses on tubes. The oxides of sulfur in the flue gas unite with that water to form sulfuric acid which is highly corrosive. In order to deter the formation of sulfuric acid, manufacturers of HRSGs attempt to configure the HRSGs such that the feedwater enters them at a temperature above the dew point for the exhaust gases.
In one configuration that achieves this end (FIG. 1), a pump draws off some of the heated feedwater at the outlet of the feedwater heater and returns it to the inlet of the feedwater heater where it mixes with the colder feedwater derived from the condenser. The temperature of the mixed feedwater is elevated to about 140xc2x0 F. This configuration requires an additional pump which operates continuously and consumes electrical energy. Apart from that, the pump requires maintenance from time to time.
In a more sophisticated configuration (FIG. 2), the feedwater heater has two sections which are located in series insofar as the flow of the exhaust gases is concerned, there being an upstream section and a downstream section. The water flows in the opposite direction, that is to say first through the downstream section and then through the upstream section. Between the two sections the water flows through a heat exchanger that is external to the gas flow. The feedwater from the condenser also passes through the external heat exchanger before entering the first section of the heater. The heat exchanger elevates the temperature of the feedwater from the condenser to at least 140xc2x0 F. before the water enters the downstream section of the feedwater heater, so no condensation occurs on the tubes of that section. Since the feedwater entering the upstream section, after having passed through the heat exchanger, must be at least 140xc2x0 F. to avoid producing condensation, the temperature differential between the exhaust gases and the tubes at the outlet of the downstream section is relatively smallxe2x80x94or, in other words, the downstream section is tightly pinched at its outlet. This requires a greater surface area to achieve the required heat transfer.
The problem of condensation in feedwater heaters or economizers is not confined solely to HRSGs installed downstream from gas turbines. Indeed, it can occur almost anywhere energy is extracted from hot gases flowing through a duct to heat the feedwater for a boiler. For example, many power plants convert the hot gases derived from the combustion of fossil fuels, such as coal or oil, directly into steam, and the boilers required for the conversion, to operate efficiently, should have feedwater heatersxe2x80x94heaters which should not produce condensation. Also, systems exist for producing steam from the hot gases derived from the incineration of waste, and they likewise have boilers including feedwater heaters that should not be subjected to condensation.
The present invention resides in a feedwater heater having two sections, each of which sees gases at essentially the same temperature at its upstream face. Feedwater enters the first section through a heat exchanger where it is heated by water flowing from the first section to the second section. The invention also resides in a process embodied in the operation of the feedwater heater and in a steam generator containing the feedwater heater.